The present invention relates to a device for localizing and two- or three-dimensional imaging of sources of gamma and/or X-rays.
The invention is applicable in particular to the following fields:                nuclear medicine;        monitoring nuclear power stations;        monitoring ports, stations, airports;        more generally, combating the smuggling of radioactive materials; and        radiography and non-destructive inspection.        
Reference can be made to the following documents:
[1] Mise en oeuvre et étude des propriétés spectrales de la gamma-caméra ISGRI [Implementation and study of the spectral properties of the ISGRI gamma camera], thesis presented and supported by Olivier Limousin on Nov. 27, 2001;
[2] The basic component of the ISGRI CdTe gamma-ray camera for space telescope IBIS on board the INTEGRAL satellite, by O. Limousin et al., Nuclear Instruments and Methods in Physics Research A 428 (1999) 216-222;
[3] Polycell: the elementary detection unit of the ISGRI CdTe & gamma-ray camera, by O. Limousin et al., Nuclear Instruments and Methods in Physics Research A 458 1-2 (2001) 551-557;
[4] A basic component for ISGRI, the CdTe gamma camera on board the Integral satellite, by M. Argues et al., IEEE Transactions on Nuclear Science, Vol. 46, No. 3, 1999, 181-186;
[5] The ISGRI CdTe gamma-ray camera: first steps, by O. Limousin et al., Nuclear Instruments and Methods in Physics Research A 442 (2000) 244-249; and
[6] Qualification model of the space ISGRI CdTe gamma-ray camera, by O. Limousin et al., Nuclear Instruments and Methods in Physics Research A 471 (2000) 174-178.
For imaging or spectro-imaging in the gamma or X-ray range from about 20 kiloelectron volts (keV) to 2 megaelectron volts (MeV), known detector systems generally make use of one of the three following techniques: coded mask imaging; “Compton” imaging; and gamma or X-ray focusing.
Coded mask imaging consists in recording on a position detector that is sensitive to gamma or X-ray photons, the shadow of a mask that is partially opaque to said photons and that presents a pattern that is known, the mask being illuminated by the gamma or X-ray source that is to be detected and localized. The position detector samples the shadow that is cast. The position of the source is reconstituted by calculation.
It is not necessary to record the energy given up by the gamma or X-ray photons in order to constitute the image, and that technique does not give information about energy. In contrast, it can be applied over very wide spectral ranges. Nevertheless, it is generally restricted to low energies, less than 1 MeV.
Its main advantages lie in good angular resolution, and in background noise being automatically subtracted, thereby improving sensitivity (i.e. improving the ratio of signal (source) over noise (environment)). Angular resolution can typically be of the order of a few minutes of arc. It is unvarying and depends on the size of the mask elements, on the capacities of the position detector to sample said elements, and on the distance between the mask and the detection plane.
Coded mask imaging systems present a height lying in the range a few centimeters to a few meters. Their field of view is restricted to a few degrees or a few tens of degrees.
The “Compton” imaging technique makes use of at least two gamma or X-ray detectors that are capable of recording the positions of interactions between an incident gamma or X-ray photon and the detectors, together with the energies given up by said photon in those positions.
The principle of this technique relies on the fact that a gamma or X-ray photon having sufficiently high incident energy presents a high probability of giving up only a fraction of that energy to an electron that is bound to the atoms of the first detector it encounters as a result of an inelastic impact, after which it diffuses towards the second detector where it gives up all or some of its residual energy.
The angle of diffusion is associated with the energy of the incident photon and with the amount of energy it gives up during its interaction with the first detector.
A “Compton” system is a spectro-imager. In such a system, the direction of the incident photon and thus the position of the emitting source are reconstituted by calculation. The angular resolution of the system and the accuracy of localization depend on the accuracy with which energy is measured and on measurement statistics (exposure time and flux from the source).
In a “Compton” system that is ideal, i.e. that provides “perfect” spectrum measurement, angle resolution is always limited by the “Doppler enlargement” effect: the diffusion angle suffers from uncertainty associated with the fact that the electron on which the photon diffuses is not at rest at the time of the impact. Angular resolution is therefore limited to about 2° to 30°.
Furthermore, “Compton” systems have a field of view that is very extended, potentially 4π steradians. The limit on the sensitivity of this type of imager comes from the processing of fortuitous coincidences that have nothing to do with Compton diffusion. In addition, such systems are compact and can be of dimensions that are as small as a few centimeters.
The principle of a spectrometric imager using gamma or X-ray focusing is based on using a gamma or X-ray lens that is fitted with crystals on which gamma or X-ray photons are diffused (Bragg diffusion).
The angular resolution of such an imager is excellent, being equal to about 1 minute of arc, however the imager is limited by a focal length that is gigantic, depending strongly on the energy of the incident photons and being equal to about 15 meters (m) at 122 keV and about 80 m at 511 keV.
In addition, imaging is direct but limited to a narrow spectral range, and the field of view of a system of this type is minuscule: it covers a few minutes of arc. Applications of the gamma or X-ray focusing technique are therefore mostly limited to astronomy.
The three techniques outlined above can be used for determining the energy and the angular localization of sources that are situated at a great distance, practically at infinity. As a result, the corresponding detection systems find their main applications in high energy astronomy.
In addition, all of those systems are either bulky or limited in terms of spectral band width, or else they provide low performance in terms of angular resolution at high energy. Furthermore, they are well adapted only to two-dimensional localization of gamma or X-ray sources that are assumed to be “at infinity”, i.e. at a distance that is at least eight times longer than the height of the telescope incorporating a system using any one of three types considered above.
Document GB 2 293 742 describes a gamma camera based on the principle of coded mask imaging and it enables a source of gamma radiation situated at a finite distance to be localized in three dimensions. The distance of the source from the camera is determined by analyzing the enlargement of the shadow of the coded mask that is projected by said source on the detector. The sensitivity of such a camera is found to be insufficient for detecting a weak gamma source presenting radiation of flux that is of the same order of magnitude as background noise, or even less.
The ISGRI gamma telescope is based on the coded mask technique, however it includes two detectors in parallel planes: consequently, at least in principle, it could be used in Compton mode. In reality, as shown in the article “Performance of the IBIS Compton mode”, SPIE 2004, 5488 page 738, by A. Segreto, the performance of the IBIS telescope in Compton mode is not satisfactory because of its low immunity to background noise.